Neuronal mechanisms contributing to long-term sensitization in Aplysia.

1.) Cellular processes that contribute to the acquisition and expression of long-term sensitization have been examined in Aplysia. The tail-siphon withdrawal reflex was studied because the neural circuit for this reflex has been well characterized. Furthermore, the sensory neurons of this neural circuit exhibit cellular changes that accompany short-term sensitization. 2.) Repeated application of noxious stimuli to the animal produces a long-lasting enhancement of reflex withdrawal of the siphon when the animal is tested with a weak stimulus to the tail. These findings confirm the existence of long-term sensitization in Aplysia, first described by Pinkser et al. (1973). 3.) Biophysical correlates of long-term sensitization were examined in the first central relay of the tail-siphon reflex circuit, the sensory neurons that innervate the animal's tail. The net outward membrane currents of these cells reduced after 24 hours as a consequence of long-term sensitization training. 4.) The intracellular signal for the induction of these changes in membrane currents was examined by intracellular injection of cAMP into individual sensory neurons. This procedure mimics at least some of the effects of sensitization training at the single-cell level. cAMP induced a long-term reduction of membrane K+ currents 24 hours after the cells were injected with cAMP. The membrane currents reduced by cAMP were similar to those reduced by long-term sensitization training. 5.) Preliminary experiments indicate that neurotransmitters and agents that induce an evaluation of cAMP in the sensory neurons also alter the incorporation of labeled amino acids into specific proteins in the sensory neurons.(ABSTRACT TRUNCATED AT 250 WORDS)